Read-only memories (ROMs) have existed for as long as there has been a need for memory for computing devices. Traditionally, ROMs have been manufactured on silicon integrated circuits of millimeter dimensions for use in microprocessor-based commercial and military applications. However, applications exist for ROMs that are manufactured on large-area flexible substrates made from plastic, paper, or cardboard. Here “large-area” means larger than typical millimeter-scale silicon integrated circuit dimensions. It is also desirable for the stored digital information in a large-area ROM to be easily programmable at the end of the fabrication process, perhaps using a high-speed printer. For example, one application of this type of ROM is for storing the serial number of a product in an electronic, machine-readable form on the product packaging.
Fabricating a ROM on a large-area, flexible substrate is technologically challenging, since standard ROM architectures used in integrated circuits place transistors in each ROM cell. While fabrication of transistors on large-area substrates is possible, with current technology it is only possible on rigid substrates such as large sheets of glass, and it requires very capital-intensive, technologically advanced process lines. This is the technology used for active-matrix displays, with fabrication facilities costing about $4 billion. Fabricating transistors on flexible substrates, even small ones, is very difficult, and is only done in research labs; no production process exists at this time.
An alternative approach for manufacturing ROMs that does not require transistors is known in the art. Instead of transistors, this approach uses nonlinear two-terminal elements such as a diodes. For example, Schottky diodes have been fabricated in laboratory environments using a metal layer and a conducting polymer layer, which is compatible with a flexible substrate. This approach is shown in FIG. 1. The ROM 2 includes a plurality of row conductors 4a-4n which are laid out in a separate plane, possible perpendicular to, but not contacting a plurality of column conductors 6a-6n. A ROM cell 8 includes a segment of one of the row conductors 4b, a segment of one of the column conductors 6b, and a diode 10. The anode 12 of the diode 10 is connected to a contact 14 on the row conductor 4b and the cathode 16 is connected to a contact 18 on the column conductor 6b. A zero or one is permanently encoded into the ROM cell 8 during fabrication by the presence or absence of the diode 10 between the row conductor 4b and the column conductor 6b. The information in a particular row, e.g. 4a, is read out by voltage sensing amplifiers 20a, 20n, by applying a positive voltage pulse 22 on the row conductor 4a, and sensing the voltage emanating from all the buffer amplifiers 20a-20n on all columns. The pull-down resistor 11 on each of the columns 6a-6n maintains that column at a fixed voltage of zero volts unless it is pulled up to a positive voltage by a diode in the selected row. The reason a diode or other nonlinear element is used in the prior art is to prevent current from flowing from a column into unselected rows, which would keep the column from being pulled up to a full positive voltage, but would hold it near zero volts.
Because diodes can be fabricated more easily than transistors, fabrication of a diode-based ROM on a large-area, flexible substrate is somewhat easier than fabrication of a transistor-based ROM on a large-area, flexible substrate. Nevertheless, fabrication of diodes on flexible substrates, even over a small area, is very difficult, and at this time is only performed at the research level.
As such, there is a need in the art for a ROM whose elements can be manufactured cost-effectively on a large-area, flexible substrate.